Powdered niobium, sintered body thereof, capacitor using the sintered body and production method of the capacitor

ABSTRACT

The present invention relates to a powdered niobium for a capacitor, characterized in that the content of each of the elements such as iron, nickel, cobalt, silicon, sodium, potassium and magnesium is about 100 ppm by weight or less or that the total content thereof is about 350 ppm by weight or less is used, a sintered body thereof, a sintered body comprising niobium monoxide crystal and/or diniobium mononitride crystal, a capacitor using the sintered body and the production method of the capacitor. 
     A capacitor using the above described niobium sintered body has a large capacity per the unit weight, a good specific leakage current value and excellent high temperature property.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 09/673,303filed on Oct. 16, 2000, now U.S. Pat. No. 6,387,150 B1, which is aNational Stage application of PCT/JP00/00858 filed Feb. 16, 2000 andwhich claims benefit pursuant to 35 U.S.C. § 119(e)(i) of the filingdates of Provisional Application 60/121,692 filed Feb. 25, 1999, andProvisional Application 60/124,665 filed Mar. 16, 1999 pursuant to 35U.S.C. § 111(b), the disclosures of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a powdered niobium for a capacitorhaving a large capacity per unit weight and good specific leakagecurrent property, a sintered body of the powdered niobium, a capacitorusing the sintered body and production method of the capacitor.

DESCRIPTION OF RELATED ART

Capacitors used for electronic instruments such as portable telephoneand personal computer are demanded to be compact and have a largecapacity. Among these capacitors, a tantalum capacitor is preferablyused because it has a large capacity for the size and exhibits goodperformance. In this tantalum capacitor, a sintered body of powderedtantalum is generally used for the anode moiety. In order to increasethe capacity of the tantalum capacitor, it is necessary to increase theweight of sintered body or to use a sintered body increased in thesurface area by pulverizing the powdered tantalum.

The former method of increasing the weight of sintered body is naturallyaccompanied by enlargement of the capacitor size and the requirement fordownsizing cannot be satisfied. On the other hand, in the latter methodof pulverizing the powdered tantalum to increase the surface area, thepore size of tantalum sintered body is reduced or closed pores areincreased at the stage of sintering, therefore, a cathode agent can bedifficultly impregnated in the after process. As a means for solvingthese problems, a capacitor using a sintered body of a powdered materialhaving a dielectric constant larger than the tantalum is being studied.Examples of such a material having a larger dielectric constant includeniobium and titanium.

However, conventional capacitors using a sintered body of such amaterial are disadvantageous in that the specific leakage currentproperty is greatly dispersed and not satisfactory by any means. Thereis no capacitor but meets the criterion that the specific leakagecurrent value in actual measurement is 10 nA/μF·V or less if thecapacitor is produced by preparing a sintered body using the tantalumpowder, oxidizing the sintered body electrolytically, and then combiningwith the counter electrode. However, in capacitors using conventionalpowderd niobium and titanium, the specific leakage current values aregreatly dispersed and there are many cases which exceed this value.

Furthermore, conventional capacitors using a sintered body of such amaterial are deficient in the high-temperature property and are not putinto practical use. Because, when a sintered body is electrolyticallyoxidized and then combined with counter electrode to manufacture acapacitor, capacity property at high temperature usually falls within±20% in the case of a sintered body using powdered tantalum, however, insome sintered bodies using conventional powdered niobium, capacityproperty at high temperature does not fall within ±20%.

Therefore, capacitors using a niobium sintered body and a titaniumsintered body must be estimated to have low reliability also at roomtemperature and are duly judged deficient in the service life, thuscannot be used in practice.

SUMMARY OF THE INVENTION

The present inventors have made an intensive study on a capacitor usinga sintered body of niobium. As a result, the present inventors havedeveloped a powdered niobium with a lower content of impurity elementswhich is capable of providing a capacitor having a small dispersion inthe specific leakage current value. Furthermore, the present inventorshave found that a capacitor having good high temperature property isobtained when a crystal of a given niobium compound is comprised in aniobium sintered body, and then accomplished the present invention basedon these findings.

Namely, the present invention relates to the following powdered niobiumfor capacitor, sintered body thereof, capacitor using the same andproduction method of the capacitor.

-   1) A powdered niobium for a capacitor, containing elements such as    iron, nickel, cobalt, silicon, sodium, potassium and magnesium,    wherein an amount of each element is 100 ppm by weight or less.-   2) A powdered niobium for a capacitor, containing elements such as    iron, nickel, cobalt, silicon, sodium, potassium and magnesium,    wherein the total amount of the elements is 350 ppm by weight or    less.-   3) A powdered niobium for a capacitor, containing elements such as    iron, nickel, cobalt, silicon, sodium, potassium and magnesium,    wherein an amount of each element is 100 ppm by weight or less and    the total amount of the elements is 350 ppm by weight or less.-   4) The powdered niobium for a capacitor described in any one of the    above 1) to 3), which contains at least one of niobium nitride,    niobium carbide and niobium boride.-   5) A sintered body for a capacitor using a powdered niobium    described in any one of the above 1) to 4).-   6) A niobium sintered body for a capacitor, comprising at least one    of niobium monoxide crystal and a diniobium mononitride crystal.-   7) The niobium sintered body for a capacitor according to the above    6), wherein the content of niobium monoxide crystal is from 0.1 wt %    to 20 wt %.-   8) The niobium sintered body for a capacitor according to the above    6), wherein the content of diniobium mononitride crystal is from 0.1    wt % to 20 wt %.-   9) A capacitor comprising one party electrode assigned to the    niobium sintered body described in any one of the above 5) to 8),    the other party electrode and a dielectric material interposed    between the two electrodes.-   10) The capacitor according to the above 9), wherein the dielectric    material is tantalum oxide, niobium oxide, polymer material, or    ceramics compound.-   11) The capacitor according to the above 10), wherein the dielectric    material is niobium oxide formed by chemical forming on a niobium    sintered body.-   12) A process for producing a capacitor, comprising preparing the    second electrode opposing on the dielectric material, after forming    the dielectric material on the niobium sintered body (first    electrode) described in any one of the above 5) to 8).-   13) The process for producing a capacitor according to the above    12), wherein the dielectric material is tantalum oxide, niobium    oxide, polymer material, or ceramics compound.-   14) The process for producing a capacitor according to the above    13), wherein the dielectric material is niobium oxide formed on a    niobium sintered body by chemical forming.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have found that a capacitor having a smalldispersion in the specific leakage current value can be obtainedprovided that a powdered niobium for a capacitor contains impurityelements such as iron, nickel, cobalt, silicon, sodium, potassium andmagnesium each in an amount of about 100 ppm by weight or less, andthese elements in a total amount of 350 ppm by weight or less.

The reason for obtaining such a result is not clear in detail, but it isassumed that when the impurity elements as impurities such as iron,nickel, cobalt, silicon, sodium, potassium and magnesium present morethan some content in a powdered niobium, the impurity elements enter thedielectric layer when a capacitor is manufactured using the powderedniobium containing the elements and cause abnormal concentration ofcharges when a voltage is applied, as a result, the specific leakagecurrent value of the capacitor is dispersed.

Furthermore when the niobium sintered body comprises a given niobiumcompound crystal, the high temperature property of a capacitor isimproved. The reason is supposed as below.

Namely, the niobium-sintered body is inferior in the stability of theoxide dielectric film as compared with the tantalum-sintered body. Manyreasons may be considered for this, but in one thinking, heat straindeveloped at a high temperature due to difference between thecomposition of the oxide dielectric film and the composition of theniobium sintered body seems to accelerate the deterioration of the oxidedielectric film. However, this heat strain seems to mitigate when aniobium monoxide crystal and/or a diniobium mononitride crystal iscontained in a niobium sintered body, as a result, a capacitor using theniobium sintered body is improved in the high temperature property.

In the present invention, the raw material of powdered niobium may be acommonly available material.

For example, the powdered niobium which can be used may be obtained byreducing potassium niobium halide with magnesium or sodium, reducingniobium fluoride with sodium, electrolyzing potassium niobium fluoridewith a molten salt (NaCl+KCl) on a nickel cathode, or introducinghydrogen into a metal niobium ingot, and then pulverizing the product.

The powdered niobium obtained by such a method contains impurityelements derived from the raw material, reducing agent or instrumentused. Representative examples thereof are the impurity elements such asiron, nickel, cobalt, silicon, sodium, potassium and magnesium.

In the present invention, by adjusting the content of each impurityelement to 100 ppm by weight or less, preferably 70 ppm by weight orless, more preferably 30 ppm by weight or less, the dispersion of thespecific leakage current value can be reduced.

Alternatively, by adjusting the total content of the impurity elementsto 350 ppm by weight or less, preferably 300 ppm by weight or less, morepreferably 200 ppm by weight or less, the dispersion of the specificleakage current value can be reduced.

In the present invention, washing method is applied in order to adjustthe content of each of the above elements to the desired ppm by weightor less. For example, the content of each element can be adjusted to 100ppm by weight or less by repeatedly washing the powdered niobiumobtained above using an acid containing at least one of hydrofluoricacid, nitric acid, sulfuric acid and hydrochloric acid and an alkali orusing the acid, an alkali and an aqueous hydrogen peroxide, in sequenceor in combination.

For another example, the powdered niobium is thoroughly washed withsulfuric acid, then neutralized by an alkali to remove the sulfuric acidand repeatedly washed with water. In the case of using nitric acid, acombination use thereof with aqueous hydrogen peroxide is advantageousbecause the powder can be prevented from being oxidized by the nitricacid.

As the washing method, a method where the powder is stirred in theabove-described reagent for a time period long enough to extract theimpurities until each impurity content reaches a predetermined amount orless, may be used.

The present inventors have found that when the powdered niobiumcomprises a compound partly bonded to at least one of nitrogen, carbonand boron, the leakage current property is improved.

Such compounds are niobium nitride, niobium carbide and niobium boride,which are a bonded product with nitrogen, carbon or boron. Thesecompounds may be contained any of them or two or three thereof.

The content of niobium nitride, niobium carbide and niobium boridevaries depending on the shape of powdered niobium, however, in the caseof a powder having an average particle size of from 0.2 μm to 30 μm, theamount bonded is from 50 ppm to 200,000 ppm, preferably from 300 ppm to20,000 ppm. If the amount bonded is less than 50 ppm, the leakagecurrent property is deteriorated, whereas if it exceeds 200,000 ppm, thecapacity property is deteriorated and the capacitor manufactured cannotbe used in practice.

The nitriding for forming niobium nitride may be any of liquidnitriding, ion nitriding, gas nitriding and the like, however, gasnitriding in a nitrogen gas atmosphere is preferred because this issimple and easy.

The gas nitriding in a nitrogen gas atmosphere is achieved by allowingthe powdered niobium to stand in a nitrogen atmosphere. A powderedniobium having an objective nitrogen content can be obtained by allowinga powdered niobium to stand in a nitriding atmosphere at a temperatureof 2,000° C. or less for tens of hours or less. In general, as thetemperature is higher, the nitriding is achieved within a shorter time.When a powdered niobium is allowed to stand in a nitrogen atmosphere atroom temperature for tens of hours, a powdered niobium having a nitrogencontent of tens of ppm by weight can be obtained.

The carbonization for forming niobium carbide may be any of gascarbonization, solid phase carbonization and liquid carbonization. Thecarbonization may be achieved by allowing a powdered niobium to standtogether with a carbon source, for example, a carbon material or anorganic material containing carbon such as methane, at 2,000° C. or lessunder reduced pressure for from several minutes to tens of hours.

The boriding for forming niobium boride may be any of gas boriding andsolid phase boriding. For example, the boriding may be achieved byallowing a powdered niobium to stand together with a boron source, forexample, a boron pellet or a boron halide such as trifluoroboron, at2,000° C. or less under reduced pressure for from several minutes totens of hours.

The niobium sintered body for a capacitor of the present invention isproduced by sintering the above-described powdered niobium. As anexample, however the production process of the sintered body is by nomeans limited to this example, a powdered niobium is pressure formedinto a predetermined shape and then heated at from 500° C. to 2,000° C.under from about 1 to 10⁻⁶ Torr for from several minutes to severalhours.

For improving the high temperature property in the present invention,the sintered body of niobium may contain niobium monoxide crystal (NbO)and/or diniobium mononitride crystal(Nb₂N).

The sintered body comprising niobium monoxide crystal or diniobiummononitride crystal can be prepared by mixing with fine powder (averageparticle size: approximately from 0.1 to 100 μm) of the above-describedcrystal(s) with powdered niobium in advance, before sintering.

In the case where the powdered niobium used in the present invention isa partially nitrided powdered niobium, a part or the whole of thenitrided powered niobium may be crystallized by controlling theconditions at the sintering of the powder compact, such as temperaturerising rate, maximum temperature, residence time at the maximumtemperature and temperature falling rate, to obtain diniobiummononitride crystal.

The powdered niobium for use in the present invention is one of valvemetals the same as aluminum and tantalum, therefore, the surface thereofis covered with an oxide in air. The oxygen amount on the surface variesdepending on the average particle size of powdered niobium. In the caseof powdered niobium having an average particle size of from 3 to 30 μm,the oxygen amount is usually from 500 to 30,000 ppm by weight. In thispowdered niobium having thereon an oxide, a part or the whole of theoxide may be crystallized, similarly to the aforementioned case ofcompacting and then sintering partially nitrided powdered niobium, bycontrolling the conditions at the sintering, such as temperature risingrate, maximum temperature, residence time at the maximum temperature andtemperature falling rate, to obtain niobium monoxide crystal.

In the case of using the crystallization technique by the control ofsintering conditions, when the relationship between the above-describedsintering conditions and the amount of each crystal obtained from thenitride and/or oxide is detected in a preliminary experiment, thesintered body as described above containing a predetermined amount ofniobium monoxide and/or diniobium mononitride crystal may be obtainedwith a reduced or zero amount of niobium monoxide crystal and/ordiniobium mononitride crystal previously mixed with powdered niobium.

The content of niobium monoxide is preferably from 0.1 wt % to 20 wt %,more preferably from 0.1 wt % to 10 wt %. The content of diniobiummononitride is preferably from 0.1 wt % to 20 wt %, more preferably from0.1 wt % to 10 wt %. If each content exceeds 20 wt %, the initialcapacity value C₀ rather decreases and this is not preferred.

A capacitor can be produced with two electrodes and a dielectricmaterial interposed between two electrodes, one part electrode (firstelectrode) being the sintered body, and the other electrode(secondelectrode) being on the dielectric material.

Examples of the dielectric material which can be used for the capacitorinclude tantalum oxide, niobium oxide, polymer materials and ceramiccompounds. In the case of using tantalum oxide as the dielectricmaterial, the tantalum oxide may be formed also by attaching a complexcontaining tantalum, such as alkoxy complex or acetylacetonato complex,to the electrode and then hydrolyzing and/or thermally decomposing thecomplex.

In the case of using niobium oxide as the dielectric material, theniobium oxide may be formed also by chemically forming the niobiumsintered body as one part electrode in an electrolytic solution or byattaching a complex containing niobium, such as alkoxy complex oracetylacetonato complex, to the electrode and then hydrolyzing and/orthermally decomposing the complex. In this way, by chemically formingthe niobium sintered body in an electrolytic solution or hydrolyzingand/or thermally decomposing a niobium-containing complex on the niobiumelectrode, a niobium oxide dielectric may be formed on the niobiumelectrode. The chemical formation of niobium electrode in anelectrolytic solution is usually performed using an aqueous protonicacid solution, for example, a 0.1% aqueous phosphoric acid or sulfuricacid solution.

In the case where a niobium oxide dielectric is formed by chemicallyforming the niobium electrode in an electrolytic solution, the capacitorof the present invention is an electrolytic capacitor and the niobiumelectrode side acts as an anode. In the case where the dielectric isformed by decomposing a complex, the electrode has theoretically nopolarity and may be used either as an anode or a cathode.

In the case of using a polymer material as the dielectric material, asdescribed, for example, in JP-A-7-63045 (the term “JP-A” as used hereinmeans an “unexamined published Japanese patent application”), a methodof introducing a monomer in the gas or liquid state into pores or voidsof a metal and polymerizing it, a method of introducing a polymermaterial after dissolving it in an appropriate solvent, or a method ofmelting and then introducing a polymer material may be used. Examples ofthe polymer material include fluororesin, alkyd resin, acrylic resin,polyester resin such as polyethylene terephthalate, vinyl-type resin,xylylene resin and phenol resin.

For forming a ceramic compound into a dielectric material, as described,for example, in JP-A-7-85461, a method of producing a perovskitecompound on the surface of a metal having pores or voids may be used.Specific examples of the perovskite compound include BaTiO₃, SrTiO₃ andBaSnO₃.

The other part electrode of the capacitor of the present invention isnot particularly limited and, for example, at least one compoundselected from an electrolytic solution, an organic semiconductor and aninorganic semiconductor, which are all known in the art of aluminumelectrolytic capacitor, may be used.

Specific examples of the electrolytic solution include a mixed solutionof dimethylformamide and ethylene glycol having dissolved therein about5 wt % of an isobutyltripropylammonium borotetrafluoride electrolyte,and a mixed solution of propylene carbonate and ethylene glycol havingdissolved therein about 7 wt % of tetraethylammonium borotetrafluoride.

Specific examples of the organic semiconductor include an organicsemiconductor comprising benzopyrroline tetramer and chloranile, anorganic semiconductor mainly comprising tetrathiotetracene, an organicsemiconductor mainly comprising tetracyanoquino-dimethane, and anorganic semiconductor mainly comprising an electrically conductingpolymer obtained by doping a dopant into a polymer comprising 2 or moreof repeating unit represented by formula (1) or (2) shown below.

(In the formulae, R¹ to R⁴, which may be the same or different, eachrepresents hydrogen, an alkyl group having from 1 to 6 carbon atoms oran alkoxy group having from 1 to 6 carbon atoms, X represents an oxygenatom, a sulfur atom or a nitrogen atom, R⁵ is present only when X is anitrogen atom and represents hydrogen or an alkyl group having from 1 to6 carbon atoms, and R¹ and R² or R³ and R⁴ may be combined with eachother to form a ring.)

Specific examples of the inorganic semiconductor include an inorganicsemiconductor mainly comprising lead dioxide and manganese dioxide, andan inorganic semiconductor comprising triiron tetraoxide. Thesesemiconductors may be used either individually or in combination of twoor more thereof.

Examples of the polymer comprising the repeating unit represented byformula (1) or (2) include polyaniline, polyoxyphenylene,polyphenylenesulfide, polythiophene, polyfurane, polypyrrole,polymethylpyrrole and derivatives of these polymers.

Out of these organic or inorganic semiconductors, when a semiconductorhaving conductivity of from about 10⁻² to about 10³ S·cm⁻¹ is used, thecapacitor manufactured can be more reduced in the impedance and can bemore increased in the capacity at a high frequency.

In the case when the other part electrode (second electrode) is a solid,a conducting layer may be formed on the electrode to improve theelectric contact with external leading terminal (e.g. Lead-frame).

The conducting layer can be formed, for example, by solidifyingconducting paste, plating, deposition of metal, forming conducting resinfilm having heat-resisting etc. As a conducting paste, silver paste,copper paste, aluminium paste, carbon paste or nickel paste ispreferable. These may be used either individually or in combination oftwo or more thereof. In a case of using two or more, it may be mixed orsequentially laminated independently. The conducting paste after appliedis stood in air or solidified by heating. As a plating method,nickel-plating, copper-plating, silver-plating, or aluminium-plating isapplied. The metal for the deposition is aluminium, nickel, copper orsilver etc.

Specifically a capacitor is fabricated, for example, by sequentiallylaminating carbon paste and silver paste on the second electrode andsealing the laminate with a material such as epoxy resin. This capacitormay have a niobium or tantalum lead, which is sintering formedintegrally with the niobium sintered body or afterward welded.

The capacitor manufactured in the present invention, having a structureas described above, can be capacitor products for various uses byfabricating with resin molding, resin-case, outer case of metal, resindipping or packing by laminate-film.

In the case where the second electrode is a liquid, a capacitor asdescribed above consisting of two electrodes and a dielectric materialis housed, for example, in a can electrically connected to the secondelectrode to accomplish the capacitor. In this case, the niobiumsintered body electrode side is designed to be insulated from the canusing an insulating rubber or the like at the same time when guidedoutside through the niobium or tantalum lead.

By manufacturing a capacitor as described in the foregoing, a largecapacity per unit weight and a small dispersion in the specific leakagecurrent value can be attained and the probability of the specificleakage current value exceeding 10 [nA/μF·V] can be reduced, as aresult, the capacitor obtained can have good specific leakage currentproperty and high reliability.

Furthermore, when a niobium monoxide crystal and/or a diniobiummononitride crystal is contained in a niobium sintered body, a capacitorwhich has an improved high temperature property can be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

Specific examples of the present invention are described in greaterdetail below. But the present invention is not limited by the examples.

The weight of impurity elements contained in the powdered niobium wasdetermined by an atomic absorption analysis.

The amount of niobium monoxide crystal and/or diniobium mononitridecrystal in the niobium sintered body was calculated using the weight ofeach crystal contained in the mixed powder before sintering and theratio in the 2θ diffraction intensity measured by X-ray diffractionbetween the mixed powder before sintering and the pulverized powder ofthe sintered body.

The specific leakage current value as used herein is defined as a valueobtained by dividing the leakage current value 1 minute after theapplication of a rated voltage (6.3V) at room temperature by the productof capacity (measured at room temperature and 120 kHz) and ratedvoltage. The capacitor in which the specific leakage current value is 10[nA/μF·V] or less is rated non-defective, and the product is evaluatedby the rate in number of non-defecdtive product using 50 samples.

The high-temperature property of the capacitor was evaluated by theratio (C−C₀)/C₀ wherein, C₀ is an initial capacity at room temperatureand C is the capacity after the capacitor was left standing for 2,000hours in an atmosphere of 105° C. at an applied voltage and thenreturned to the room temperature, and in the case where this ratio fallswithin ±20%, the product was rated non-defective The evaluation was madeby the ratio in number of non-defective using 50 samples.

TEST EXAMPLES 1 TO 6

Niobium pentachloride and magnesium were thoroughly dried under vacuumat 80° C. and charged into a nickel crucible to allow the reductionreaction to proceed at 800° C. for 40 hours in an argon atmosphere. Thereduced product was cooled, washed with water to remove magnesiumchloride, washed with sulfuric acid, again washed with water and thendried under vacuum. 120 g of the dried product was placed in an aluminapot containing silica alumina balls and pulverized by ball milling toobtain powdered niobium (average particle size: 5 μm). 20 g of thepowdered niobium at this stage was designated as Test Example 1. 100 gof the powdered niobium remaining was immersed in a 1:1 mixed solutionof hydrofluoric acid and nitric acid and stirred. During the stirring,20 g of the solution was sampled every 1 hour and the solution sampleswere thoroughly washed with water until the washing water reached a pHof 7 and then dried under vacuum to obtain powdered niobium samples ofTest Examples 2 to 6 each in an amount of 20 g.

From the powdered niobium of each Test Example, 50 compacts each in asize of 3 mm×4 mm×1.8 mm were manufactured. Subsequently, the compactswere left standing in vacuum of 5×10⁻⁵ Torr at a maximum temperature of1,200° C. for 30 minutes to obtain sintered bodies. These sinteredbodies each was electrolytically formed (20V) in a 0.1% aqueousphosphoric acid solution to form a dielectric oxide film on the surfacethereof. Thereafter, each sintered body was immersed in an aqueousmanganese nitrate solution and then heated at 220° C. By repeating thisoperation of immersing and then heating, a manganese dioxide layer asthe other part electrode layer was formed on the dielectric oxide film.Then, a carbon layer and a silver paste layer were sequentiallylaminated thereon, placed on a lead frame, and the entire body wassealed with epoxy resin. In this way, 50 units of chip-type capacitorswere manufactured. The evaluation results are shown in Table 2.

TEST EXAMPLES 7 TO 9

Capacitors were produced in the same manner as in Test Example 6 exceptfor forming the other part electrode (the second electrode) by themethod shown in Table 1. The evaluation results are shown in Table 2.

TABLE 1 Other Part Electrode Method for Forming (the second electrode)Electrode Test Mixture of lead dioxide Oxidation reaction in a Example 7and lead sulfate (lead lead acetate solution dioxide: 94 wt %) wasrepeated. Test Chloranile complex of Immersing in a solution Example 8tetrathiotetracene of compound for other part electrode and then dryingwere repeated Oxidation reaction in a Test Polypyrrole doped withpyrrole solution was Example 9 aromatic sulfonic acid repeated.

TEST EXAMPLES 10 TO 15

Potassium niobium fluoride and sodium were thoroughly dried under vacuumat 80° C. and charged into a nickel crucible to allow the reductionreaction to proceed at 1,000° C. for 20 hours in an argon atmosphere.The reduced product was cooled, washed with water to remove potassiumfluoride and sodium fluoride, washed with sulfuric acid, again washedwith water and then dried under vacuum. 120 g of the dried product wasplaced in an alumina pot containing silica alumina balls and pulverizedby ball milling to obtain powdered niobium (average particle size: 4μm). 20 g of the powdered niobium at this stage was designated as TestExample 10. 100 g of the powdered niobium remaining was immersed in a3:2 mixed solution of nitric acid and aqueous hydrogen peroxide andstirred. During the stirring, 20 g of the solution was sampled every 1hour and the solution samples were thoroughly washed with water untilthe washing water reached a pH of 7 and then dried under vacuum toobtain powdered niobium samples of Test Examples 11 to 15 each in anamount of 20 g.

From the powdered niobium of each Test Example, 50 capacitors weremanufactured in the same manner as in Test Example 1. The evaluationresults are shown in Table 2.

TEST EXAMPLES 16 TO 18

20 g of powdered niobium obtained in the same manner as in Test Example10 was placed in an alumina ceramic-made boat, and the boat was chargedinto an SUS304 tube and left standing at 400° C. for 3 hours in anitrogen atmosphere to obtain partially nitrided powdered niobium havinga nitrogen content of about 2,500 ppm by weight. This was designated asthe powdered niobium of Test Example 16. 20 g of the powdered niobiumobtained in the same manner as in Test Example 10 was placed in a carboncrucible, left standing at 1,500° C. for 30 minutes under reducedpressure (about 5×10⁻⁵ Torr) in a molybdenum furnace, cooled at roomtemperature and then pulverized using a vortex mill. This was designatedas the powdered niobium of Test Example 17. 20 g of the powdered niobiumobtained in the same manner as in Test Example 17 was further nitridedin the same manner as in Test Example 16 to obtain partially carbonizedand partially nitrided powdered niobium having a carbon content of about1,000 ppm by weight and a nitrogen content of about 2,000 ppm by weight.This was designated as Test Example 18. Each of the thus-obtainedpowdered niobium samples of Test Examples 16 to 18 was washed and usedto manufacture capacitors in the same manner as in Test Example 15. Theevaluation results are shown in Table 2.

TEST EXAMPLES 19

Except that platinum crucible with 1 g of boron pellet therein was usedinstead of a carbon crucible with 20 g of powdered niobium therein andthat the standing operation was carried out not at 1500° C. but at 1000°C., the operation was conducted in the same manner as in Test Example 17to obtain a partially borated powdered niobium having about 850 ppm byweight boron element.

TEST EXAMPLES 20 TO 24

A 60-mmφ niobium ingot was charged into an SUS304-made reactor and afteronce evacuating the system by vacuumizing (about 6×10⁻⁴ Torr), thetemperature was elevated to 800° C. Subsequently, hydrogen wasintroduced, the temperature was raised to 350° C. and further hydrogenwas continuously introduced for 50 hours. The hydrogenated niobium lumpwas cooled and then pulverized in an SUS304 made pot containingiron-made balls. The resulting pulverized product was placed in theSUS304-made reactor used above and again hydrogenated under theabove-described conditions. The resulting hydride was placed togetherwith water and zirconia balls in an iron-made wet pulverizer (trade name“Attritor,” product of MITSUI MINING Co., LTD) and wet pulverized.Thereafter, the powder was washed with sulfuric acid and then withwater, and dried under vacuum to obtain about 100 g of powdered niobium(average particle size: 3 μm). The powdered niobium at this stage wasdesignated as Test Example 20. A powdered niobium treated in the samemanner as in Test Example 20 was stirred in a 1:1 mixed solution ofhydrofluoric acid and nitric acid. During the stirring, 20 g of thesolution was sampled every 1 hour and the solution samples werethoroughly washed with water until the washing water reached a pH of 7and then dried under vacuum to obtain powdered niobium samples of TestExample 21 to 24 each in an amount of 20 g. From the powdered niobium ofeach Test Example, 50 capacitors were manufactured in the same manner asin Test Example 1. The evaluation results are shown in Table 2.

TABLE 2 Specific Leakage Current, non- Test Impurity Elements [wtppm]defective/number Example Fe Ni Co Si Na K Mg Total of test samples  1100  100  40 180  15 15 200  650 45/50  2 80 80 40 110  10 10 80 41047/50  3 60 60 30 110  10 10 60 340 49/50  4 50 60 30 90 10 10 60 31050/50  5 50 60 30 70 10 10 60 290 50/50  6 50 50 30 60 10 10 50 26050/50  7 50 50 30 60 10 10 50 260 50/50  8 50 50 30 60 10 10 50 26050/50  9 50 50 30 60 10 10 50 260 50/50 10 110  110  50 200  110  180 20 780 46/50 11 90 90 30 120 80 130  20 560 47/50 12 90 70 20 90 20 6020 370 49/50 13 70 70 20 80 20 60 20 340 50/50 14 70 70 20 70 20 50 20320 50/50 15 50 50 20 60 20 50 20 270 50/50 16 50 50 20 60 20 50 20 27050/50 17 50 50 20 60 20 50 20 270 50/50 18 50 50 20 60 20 50 20 27050/50 19 50 50 20 60 20 50 20 270 50/50 20 190  190  110  80  5  5 10590 45/50 21 110  100  60 80  5  5 10 370 47/50 22 30 30 20 60  5  5 10160 50/50 23 20 20 20 40  5  5 10 120 50/50 24 20 20 20 30  5  5  5 10550/50

From comparison in Table 2 of Test Examples 1, 2 and 3 with TestExamples 4 to 9, Test Examples 10 and 11 with Test Examples 12 to 18,and Test Examples 20 and 21 with Test Examples 22 to 24, it is seen thatwhen the content of each element is 100 ppm by weight or less, theprobability of defectives occurring with respect to the specific leakagecurrent value can be reduced.

From comparison in Table 2 of Test Examples 1 and 2 with Test Examples 3to 9, Test Examples 10, 11 and 12 with Test Examples 11 to 18, and TestExamples 20 and 21 with Test Examples 22 to 24, it is seen that when thetotal content of impurity elements is 350 ppm by weight or less, theprobability of defectives occurring with respect to the specific leakagecurrent value can be reduced.

TEST EXAMPLES 25 TO 28

To powdered niobium (of which surface was covered with about 1.5 wt % ofnatural oxide) having a particle size of 4 μm which is obtained bypulverizing hydrogenated niobium ingot and then dehydrogenating thepulverized niobium, 2 wt % of niobium monoxide crystal (average particlesize: 2 μm) was mixed. 0.1 g of the resulting mixed powder was sampledand compacted simultaneously with a niobium lead to obtain a compacthaving a size of 3 mm×4 mm×1.8 mm. Subsequently, the compact was leftstanding in vacuum of 5×10⁻⁵ Torr for 30 minutes while raising thetemperature at a rate of 10° C./min to a maximum temperature of 1,100°C. and then the temperature was dropped at an average dropping rate ofabout 80° C./min while charging thereinto Ar gas to obtain a sinteredbody. In this way, 200 units of niobium sintered bodies were preparedand all were subjected to electrolytic formation (20 V) in a 0.1%aqueous phosphoric acid solution to form an oxide dielectric film on thesurface. Then, the sintered bodies were divided into 4 groups eachconsisting of 50 units. On the oxide dielectric film, the other partyelectrode (the second electrode) layer shown in Table 3 was formed,carbon paste and silver paste were sequentially laminated thereon, alead frame was connected and the whole was sealed with epoxy resin toprepare chip-type capacitors. In the sintered body, the amount ofniobium monoxide crystal was 2.6 wt %. The evaluation results are shownin Table 4.

TABLE 3 Other party electrode (Second electrode) Method for FormingElectrode Test Mixture of lead Oxidation reaction in a lead Exampledioxide and lead acetate solution was repeated. 25 sulfate (leaddioxide: 94 wt %) Test Chloranile complex of Immersing in a solution ofExample tetrathiotetracene compound for other party 26 electrode andthen drying were repeated. Test Polypyrrole doped with Oxidationreaction in a pyrrole Example aromatic sulfonic acid solution wasrepeated. 27 Test Manganese dioxide Thermal decomposition of Examplemanganese nitrate was repeated. 28

TEST EXAMPLE 29

Powdered niobium the same as in Test Example 25 was left standing in anitrogen atmosphere at 400° C. for 3 hours to obtain partially nitridedpowdered niobium having a nitrogen content of about 2,500 ppm by weight.To this powder, 0.5 wt % of diniobium mononitride crystal (averagecrystal size: 0.8 μm) was mixed. Using the mixture obtained, a capacitorwas produced in the same manner as in Test Example 25. In the sinteredbody, the amounts of niobium monoxide crystal and diniobium mononitridecrystal were 0.5 wt % and 0.7 wt %, respectively. The evaluation resultsare shown in Table 4.

TEST EXAMPLE 30

Powdered niobium (of which surface was covered with about 0.8 wt % ofnatural oxide) having an average particle size of 5.5 μm which isobtained by pulverizing hydrogenated niobium ingot and thendehydrogenating the pulverized niobium, was left standing in a nitrogenatmosphere at 800° C. for 3 hours to obtain partially nitrided powderedniobium having a nitrogen content of about 15,000 ppm by weight. To thispowder, 5 wt % of diniobium mononitride crystal (average crystal size:0.2 μm) was mixed. Using the mixture obtained, a capacitor was producedin the same manner as in Test Example 25. In the sintered body, theamount of diniobium mononitride crystal were 6.3 wt %. The evaluationresults are shown in Table 4.

TEST EXAMPLE 31

Powdered niobium the same as in Test Example 25 was placed in a carboncrucible, left standing at 1,500° C. for 30 minutes under reducedpressure, returned to room temperature, taken out from the crucible andpulverized in a vortex mill to obtain partially carbonized powderedniobium having a carbon content of about 1,000 ppm by weight. To thispowder, 7 wt % of niobium monoxide crystal used in Test Example 25 wasmixed. The resulting mixture was compacted in the same manner as in TestExample 25. The compact obtained was left standing for 1 hour in vacuumof 6×10⁻⁵ Torr while raising the temperature at a rate of 4° C./min to amaximum temperature of 1,200° C. and then returned to room temperatureover 10 hours without displacing the gas to obtain a niobium sinteredbody. Using the niobium-sintered body obtained, a capacitor was producedin the same manner as in Test Example 25. In the sintered body, theamount of niobium monoxide crystal was 8.3 wt %. The evaluation resultsare shown in Table 4.

TEST EXAMPLE 32

Partially carbonized powdered niobium of Test Example 31 was nitrided inthe same manner as in Test Example 29 to obtain partially carbonized andpartially nitrided powdered niobium having a carbon content of about1,000 ppm by weight and a nitrogen content of about 2,500 ppm by weight.To this powder, 0.1 wt % of powdered diniobium mononitride was mixed.Using the mixture obtained, a capacitor was produced in the same manneras in Test Example 31. In the sintered body, the amounts of niobiummonoxide crystal and diniobium mononitride crystal were 1.3 wt % and0.35 wt %, respectively. The evaluation results are shown in Table 4.

TEST EXAMPLE 33

A capacitor was produced in the same manner as in Test Example 25 exceptthat the amount of niobium monoxide crystal mixed was changed to 20 wt%. In the sintered body, the content of niobium monoxide crystal was20.5 wt %. The evaluation results are shown in Table 4.

TEST EXAMPLE 34

A capacitor was produced in the same manner as in Test Example 30 exceptthat the amount of diniobium mononitride crystal mixed was changed to 20wt %. In the sintered body, the content of niobium monoxide crystal was21.3 wt %. The evaluation results are shown in Table 4.

TEST EXAMPLE 35

A capacitor was produced in the same manner as in Test Example 30 exceptthat powdered niobium used was not partially nitrided and diniobiummononitride crystal was not mixed. The evaluation results are shown inTable 4.

TABLE 4 Evaluation of Content High-Temper- of Content of ature PropertyNiobium Diniobium Initial (non-defective Monoxide Mononitride Capacityunits)/(number Particle Crystal Crystal C₀ [μF] of samples) Size [μm][wt %] [wt %] Test Example 25 230 50/50 4 2.6 <0.1 Test Example 26 20050/50 4 2.6 <0.1 Test Example 27 200 50/50 4 2.6 <0.1 Test Example 28220 50/50 4 2.6 <0.1 Test Example 29 230 50/50 4 0.5 0.7 Test Example 30160 50/50 5.5 <0.1 6.3 Test Example 31 225 50/50 4 8.3 <0.1 Test Example32 225 50/50 4 1.3 0.35 Test Example 33 190 50/50 4 20.5 <0.1 TestExample 34 120 50/50 5.5 <0.1 21.3 Test Example 35 160 45/50 5.5 <0.1<0.1

From comparison between Test Examples 25 to 34 and Test Example 35 inTable 4, it is seen that when 0.1 wt % or more of the crystal is presentin the sintered body, the high-temperature property is more improved.Furthermore, from the comparison between Test Example 25 and TestExample 33 or between Test Example 30 and Test Example 34, it is seenthat when the amount of the crystal exceeds 20 wt %, the initialcapacity is reduced.

INDUSTRIAL APPLICABILITY

By using the niobium sintered body of the present invention for acapacitor, a capacitor having a large capacity per the unit weight and agood leakage current value can be produced. By using theniobium-sintered body comprising niobium monoxide crystal and/or adiniobium mononitride crystal, the high-temperature property of thecapacitor can be improved.

1. A powdered niobium for a capacitor comprising at least one of niobiumnitride, niobium carbide and niobium boride of 50 ppm by weight to200,000 ppm by weight and at least one element selected from the groupconsisting of iron, nickel, cobalt, silicon, sodium, potassium andmagnesium in an amount of 100 ppm by weight or less.
 2. A powderedniobium for a capacitor comprising at least one of niobium nitride,niobium carbide and niobium boride of 50 ppm by weight to 200,000 ppm byweight and at least one element selected from the group consisting ofiron, nickel, cobalt, silicon, sodium, potassium and magnesium in atotal amount of 350 ppm by weight or less.
 3. A powdered niobium for acapacitor comprising at least one of niobium nitride, niobium carbideand niobium boride of 50 ppm by weight to 200,000 ppm by weight and atleast one element selected from the group consisting of iron, nickel,cobalt, silicon, sodium, potassium and magnesium in an amount of 100 ppmby weight or less and in a total amount of 350 ppm by weight or less. 4.The powdered niobium for a capacitor as claimed in claim 1, wherein thepowdered niobium has an average particle size of 0.2 μm to 30 μm.
 5. Thepowdered niobium for a capacitor as claimed in claim 2, wherein thepowdered niobium has an average particle size of 0.2 μm to 30 μm.
 6. Thepowdered niobium for a capacitor as claimed in claim 3, wherein thepowdered niobium has an average particle size of 0.2 μm to 30 μm.
 7. Asintered body for a capacitor using the powdered niobium as described inclaim
 1. 8. A sintered body for a capacitor using the powdered niobiumas described in claim
 2. 9. A sintered body for a capacitor using thepowdered niobium as described in claim
 3. 10. A capacitor constructed byone electrode comprising the niobium sintered body as described in claim7, a second electrode and a dielectric material interposed between thetwo electrodes.
 11. The capacitor as claimed in claim 10, wherein thedielectric material is tantalum oxide, niobium oxide or a polymermaterial, or a ceramic compound.
 12. The capacitor as claimed in claim11, wherein the dielectric material is niobium oxide formed by chemicalforming on a niobium sintered body.
 13. A capacitor constructed by oneelectrode comprising the niobium sintered body as described in claim 8,a second electrode and a dielectric material interposed between the twoelectrodes.
 14. The capacitor as claimed in claim 13, wherein thedielectric material is tantalum oxide, niobium oxide or a polymermaterial, or a ceramic compound.
 15. The capacitor as claimed in claim14, wherein the dielectric material is niobium oxide formed by chemicalforming on a niobium sintered body.
 16. A capacitor constructed by oneelectrode comprising the niobium sintered body as described in claim 9,a second electrode and a dielectric material interposed between the twoelectrodes.
 17. The capacitor as claimed in claim 16, wherein thedielectric material is tantalum oxide, niobium oxide or a polymermaterial, or a ceramic compound.
 18. The capacitor as claimed in claim15, wherein the dielectric material is niobium oxide formed by chemicalforming on a niobium sintered body.
 19. A capacitor using a powderedniobium, wherein the capacitor is evaluated by calculating a ratio(C−C₀)/C₀ where C₀ is an initial capacity at room temperature and C is acapacity after the capacitor was left standing for 2,000 hours in anatmosphere of 105° C., and the ratio as calculated falls within ±20%.20. The niobium capacitor as described in claim 19, comprising a niobiumsintered body.
 21. The niobium capacitor as described in claim 20,wherein the niobium sintered body comprises at least one of niobiumnitride, niobium carbide and niobium boride of 50 ppm by weight to200,000 ppm by weight and at least one element selected from the groupconsisting of iron, nickel, cobalt, silicon, sodium, potassium andmagnesium in an amount of 100 ppm by weight or less.
 22. A capacitorcomprising the powdered niobium as claimed in claim 1 or 2, wherein theleakage current value is 10 [nA/μF·V] or less.